Organic-inorganic polyoxometalate based salts as thermoregulated phase-separable catalysts for selective oxidation of thioethers and thiophenes and deep desulfurization of model fuels

Article abstract of DOI:10.1016/j.molcata.2013.09.009

Synthesis of various organic-inorganic polyoxometalate based salts composed of sulfonated pyridinum cations and the Keggin tungstophosphate anion (PW ₁₂O₄₀^3-) and their subsequent application in the oxidation of methyl phenyl sulfide with hydrogen peroxide in aqueous media was reported. Special attention was paid to the temperature-dependent solubility of the salts as a function of the organic cation in water. Cyclic voltammetry was further used for the investigation of the redox behavior of these catalysts. Based on these properties, a novel thermoregulated phase-separable catalyst was introduced and successfully used for the selective oxidation of thioethers and thiophenes to the corresponding sulfoxides or sulfones. Catalytic oxidation of the sulfur-containing model oil (methyl phenyl sulfide, dibenzothiophene and thiophene) was also studied under optimized conditions. In both cases, excellent conversion and selectivity were obtained in relatively short reaction times. This novel catalyst showed the characteristic of homogeneous reaction at ambient temperatures and phase-separation at lower temperatures. By simple decantation, the catalyst could be easily separated from the products upon cooling of the reaction mixture and could be reused several times with high recycling efficiency.

Full text of DOI:10.1016/j.molcata.2013.09.009

Accepted Manuscript
Title: Organic-inorganic polyoxometalate based salts as
thermoregulated phase-separable catalysts for selective
oxidation of thioethers and thiophenes and deep
desulfurization of model fuels
Author: Ezzat Rafiee Sara Eavani
PII:
S1381-1169(13)00334-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.09.009
MOLCAA 8863
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
28-2-2013
28-8-2013
8-9-2013
Please citethisarticleas:E. Rafiee, S. Eavani, Organic-inorganicpolyoxometalatebased
salts as thermoregulated phase-separable catalysts for selective oxidation of thioethers
and thiophenes and deep desulfurization of model fuels, Journal of Molecular Catalysis
A: Chemical (2013), http://dx.doi.org/10.1016/j.molcata.2013.09.009
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Organic-inorganic polyoxometalate based salts as thermoregulated phase-separable
catalysts for selective oxidation of thioethers and thiophenes and deep desulfurization of
model fuels
Ezzat Rafiee*, Sara Eavani
Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran
Abstract
Synthesis of various organic-inorganic polyoxometalate based salts composed of sulfonated
pyridinum cations and the Keggin tungstophosphate anion (PW12O403-) and their subsequent
application in the oxidation of methyl phenyl sulfide with hydrogen peroxide in aqueous
media was reported. Special attention was paid to the temperature-dependent solubility of the
salts as a function of the organic cation in water. Cyclic voltammetry was further used for the
investigation of the redox behavior of these catalysts. Based on these properties, a novel
thermoregulated phase-separable catalyst was introduced and successfully used for the
selective oxidation of thioethers and thiophenes to the corresponding sulfoxides or sulfones.
Catalytic oxidation of the sulfur-containing model oil (methyl phenyl sulfide,
dibenzothiophene and thiophene) was also studied under optimized conditions. In both cases,
excellent conversion and selectivity were obtained in relatively short reaction times. This
novel catalyst showed the characteristic of homogeneous reaction at ambient temperatures and
phase-separation at lower temperatures. By simple decantation, the catalyst could be easily
separated from the products upon cooling of the reaction mixture and could be reused several
times with high recycling efficiency.
*Corresponding author. Tel./fax: +98-831-427-4559; e-mail: ezzat_rafiee@yahoo.com
Page 1 of 29
1

Keyword: Polyoxometalates; Thermoregulated catalyst; Deep desulfurization; Sulfur;
Oxidation
1. Introduction
The research and application of “Green Chemistry” principles have led to the development
of cleaner processes. In this sense, polyoxometalates (POMs) are widely-used as green
catalysts especially in H2O2-based oxidation reactions [1-6], whereas no corrosive species are
involved in the reaction system. Much attention has currently been focused on the use of the
POMs as catalysts in the selective oxidation of organic sulfur compounds [7-10], not only for
the syntheses of sulfoxides and sulfones as synthetic intermediates in pharmaceutical field
[11] but also in the oxidative desulfurization (ODS) process as an effective method to remove
of sulfur compounds from fuels and industrial effluents, in order to satisfy the new
environmental legislations [12-14].
The homogeneous POMs are remarkably efficient, but they share a common drawback:
separation and reuse of the catalyst is extremely difficult. For this reason, future practical
applications of POMs will also require methods for “catalyst engineering” to aid in catalyst
recovery and recycling. Recently, many improvements were developed for the synthesis of
new organic-inorganic salts through the different combinations of various organic cations with
POM anions [15-17]. Obviously, these new hybrids have emerged as one of the most
potentially significant fields of investigation in catalysis [18-21]. Until now, a few reports
have been published on ODS catalyzed with organic-inorganic POM-based salts [22-29].
Recent examples can be classified into two groups. One of them includes amphiphilic
catalysts that have been synthesized by combining quaternary ammonium cations and POMs
anions [22-24]. These compounds have ability to form emulsion droplets, which perform as
homogeneous catalysts in the interface of two immiscible liquids to achieve high activities
during the oxidative process. However, the de-emulsification for catalyst recovery is often
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2

challenging in a large-scale unit. Catalyst recovery in an active form suitable for recycling is
generally not feasible and the products may be contaminated with catalyst residues. This
situation often leads to the expensive purification procedures which disagree with the
development of more sustainable processes.
Recent advances in ionic liquids (ILs) research provide another route for achieving
organic-inorganic POM-based salts which have been constructed by creation of electrostatic
interactions between the organic IL cation and the POM anion (POM-IL salts) [15-17]. To our
knowledge there are few papers about the use of POM-based salts in the selective oxidation of
organic sulfur compounds and applications in the elimination of sulfur content in diesel are
quite limited. [MIMPS]PW/[Omim]PF6 and [PyPS]PW/[Omim]PF6 catalytic systems
(Scheme 1) have been used to deep desulfurization of fuels [25, 26]. However, the
disadvantages of these strategies are also obvious. While used as the reaction media, the
[Omim]PF6 is consumed in large amounts. Bearing in mind that [Omim]PF6 is fairly
expensive and easily yields white fumes of HF or hydrate precipitates. Furthermore, the
selectivity for sulfoxides and sulfones present in diesel oil is imperfect and problems
associated with catalyst recycling remains a major challenge. Therefore designing a POM-IL
system, which can combine the advantages of homogeneous and heterogeneous catalysts, is an
attractive field in the chemical research area.
One has to take into consideration the fact that physical and chemical properties of the
POM-ILs can be tuned by the change in the structure, functional group, or size of the ions
[30]. With this fact in mind, we have designed and synthesized a series of novel POM-IL
catalysts with the structure close to [PyPS]PW, including [BuPyPS]PW, [PhPyPS]PW, and
[BzPyPS]PW (Scheme 1), and investigated the catalytic activities and temperature-dependent
solubilities of these compounds in water. According to experimental findings, a novel
thermoregulated phase-separable catalyst was introduced for selective oxidation of organic
sulfur compounds in aqueous media. The desulfurization of model oil was also investigated
Page 3 of 29
3

under optimized reaction conditions. This novel POM-IL catalyst can be easily separated from
products upon cooling of the reaction solution. Thus, the problem associated with the
separation and reuse of the catalyst can be well resolved. As to the knowledge of the authors,
open literature does not show publications related with these new POM-ILs, therefore no
information is available about their activities or temperature-dependent solubilities until now.
3-
-
N
N (CH2)7CH3
PF6
N
N (CH2)3SO3H
PW12O40
3
[Omim]PF6
[MIMPS]PW
t-Bu
3-
PW12O40
3-
PW12O40
(CH2)3SO3H
N
N
(CH2)3SO3H
3
3
[PyPS]PW
[BuPyPS]PW
Ph
Ph
3-
3-
PW12O40
PW12O40
N
(CH2)3SO3H
N
(CH2)3SO3H
3
3
[BzPyPS]PW
[PhPyPS]PW
Scheme 1. POM-ILs structures.
2. Experimental
2.1. General
All reagents and solvents used in this work were obtained from Fluka, Aldrich or Merck
and were used without further purification. H3PW12O40 (PW) from Aldrich was used. UV-vis
spectra were obtained with an Agilent (8453) UV-vis diode-array spectrometer using quartz
cells of 1 cm optical path. NMR spectra were recorded on a Bruker Avance 200 MHz NMR
spectrometer with D2O as the solvent and TMS as the internal standard. IR spectra were
recorded as KBr pellets using a Shimadzu 470 spectrophotometer. Elemental analyses were
Page 4 of 29
4

performed using CHN Herause rapid model. Melting points were determined using a digital
Gallenkamp apparatus and uncorrected. Electrochemical experiments were performed using a
computer controlled μ-Autolab modular electrochemical system (Eco Chemie, Utrecht, the
Netherlands) driven with GPES software (EcoChemie). The products were analyzed using gas
chromatographic-mass spectroscopy (GC-MS) Agilent Technologies 6890N Instrument,
equipped with a capillary column 19019J-413 HP-5, and a flame ionization detector.
2.2. Catalysts preparation
The POM-IL catalysts were synthesized following the procedure in Scheme 2. Typically,
for the synthesis of [PyPS]3PW catalyst, pyridine (0.11 mol) and 1, 3-propanesultone (0.10
mol) were dissolved in toluene (30 mL) and stirred at 40 °C for 24 h under nitrogen
atmosphere. A white precipitate (PyPS) was formed. It was filtered, washed with diethyl ether
three times and dried in a vacuum. The resulting PyPS (0.06 mol) was added to an aqueous
solution of PW (0.02 mol PW in 40 mL H2O), and then the mixture was stirred at room
temperature for 24 h. Water was removed in a vacuum to give the final product as a solid.
Other catalysts were prepared accordingly.
Elemental analyses for the POM-ILs are as follows:
Calc. for [PyPS]PW: C, 8.27; H, 1.03; N, 1.20; S, 2.47. Found: C, 8.22; H, 1.05; N, 1.23; S,
2.70. Calc. for [BuPyPS]PW: C, 11.83; H, 1.65; N, 1.15; S, 2.63. Found: C, 11.85; H, 1.68; N,
1.16; S, 2.64. Calc. for [PhPyPS]PW: C, 13.57; H, 1.30; N, 1.13; S, 2.58. Found: C, 13.60; H,
1.32; N, 1.10; S, 2.59. Calc. for [BzPyPS]PW: C, 14.38; H, 1.45; N, 1.11; S, 2.56. Found: C,
14.41; H, 1.46; N, 1.12; S, 2.59.
Based on the elemental analyses data, it seems that the POM-IL salts basically consist of
[PyPS]3PW, [BuPyPS]3PW, [PhPyPS]3PW and [BzPyPS]3PW respectively. Melting points, IR
1
and H NMR data of the organic cations and the POM-ILs are given in the supplementary
information.
2.3. Solubility measurement of the POM-ILs in water
Page 5 of 29
5

Solubilities of the POM-ILs in water were measured by means of UV-vis spectroscopy
according to the procedures previously reported in literature [31]. The quantitative
determination of the amount of dissolved catalysts in water is based on the fact that the POM-
ILs have charge transfer bands at 256 ([PyPS]PW), 251 ([BuPyPS]PW), 285 ([PhPyPS]PW)
and 250 nm ([BzPyPS]PW) (see Fig. 1 in supplementary information). The calibration curve
for each POM-ILs was prepared based on standards provided by our laboratory for absorbance
values varying from 0.0 to 2.0. In this region the calibration curves were almost linear (see
supplementary information). To measure the solubility of catalysts, excess amounts of the
POM-ILs were added in water (5 mL) and vigorously stirred for 6 h at different temperatures.
An aliquot was separated (from insoluble POM-ILs) by centrifugation and diluted by a factor
of 5-20 times. Then the absorption value of these dilution solutions were measured, and the
amount of dissolved POM-IL can be calculated using calibration curves, considering the
diluted factor.
2.4. Voltammetry experiments
The electrochemical reduction of the POM-ILs (5 × 10-4 M) was studied in phosphate
buffer (0.1 M), pH = 1. A glassy carbon electrode was used as the working electrode and
Ag/AgCl as the reference electrode. Cyclic voltammograms (v, 0.1 Vs-1) were obtained at
ambient temperature (20±5 °C).
2.5. Catalytic reactions
2.5.1. Reaction progress monitoring
Progress of model reactions was monitored by UV-vis spectroscopy which measured the
decrease in the absorbance of methyl phenyl sulfide (MPS) or dibenzothiophene (DBT) during
the reaction. Samples (100 μL) were taken out from the reaction mixture at different reaction
times and immediately treated with 1 mL of distilled water and shaken for few minutes. Later,
1 mL of n-hexane was added, and this mixture was again shaken and agitated to develop two
phases: the non-polar phase containing MPS or DBT and the polar phase containing catalyst,
Page 6 of 29
6

H2O2 and corresponding sulfoxide and/or sulfone. The organic phase was separated and
analyzed by UV-vis spectroscopy and the product was extracted using Et2O from aqueous
phase and analyzed after catalyst separation. The solution absorbance was measured at 250 nm
for MPS and 230 nm for DBT. All the measurements were repeated at least three times and
the reported values are averages of the individual runs.
2.5.2. Oxidation of liquid sulfides (Table 1, entries 1-7)
To a stirred mixture of the sulfide (5 mmol), and catalyst (0.05 g, 0.25 mol%) in water (10
mL), 30% aq. H2O2 (0.56 mL, 5mmol) was added in one portion. The slurry was stirred at
room temperature for 20 min. The reaction mixture was cooled to 10 °C, and the catalyst was
separated by filtration. The corresponding sulfoxide product was extracted with Et2O from the
reaction mixture. Evaporation of the solvent afforded the crude product. The crude product
was purified by column chromatography on silica gel using EtOAc/hexane as eluent (method
a).
Similar method was utilized to produce sulfones. In this case 2.2 mL of 30% aq. H2O2 (20
mmol) and 0.15 g of catalyst (0.75 mol%) were utilized (method b).
2.5.3. Oxidation of solid sulfur compounds (Table 1, entries 7-10)
To a stirred suspension of the substrate (5 mmol), catalyst (0.2 g, 1.0 mol%) in H2O: EtOH
(7:3 v/v, 10 mL), 30% aq. H2O2 (1.1 mL, 10 mmol) was added in one portion. The slurry was
stirred at 60 ºC for 45 min. The reaction mixture was cooled to 10 °C, and the catalyst was
separated by filtration. The corresponding sulfoxide product was extracted with Et2O from the
reaction mixture. Evaporation of the solvent afforded the crude product. The crude product
was purified by column chromatography on silica gel using EtOAc/hexane as eluent (method
c). Similar method was utilized to produce sulfones. In this case 3.4 mL of 30% aq. H2O2 (30
mmol) was used for 60 min (method d).
2.5.4. Oxidative desulfurization of model oil
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7

Two model oils were used: (1) Model fuel containing DBT (500 ppm means 500 mg of
DBT per 1 L of n-decane), MPS (250 ppm) and thiophene (250 ppm) in 5 mL of n-decane
with the corresponding total sulfur concentration 1000 ppm and (2) Model fuel containing
DBT in 5 mL of n-decane with the sulfur concentration 100 ppm. The oxidation reaction was
carried out using 4.0 μmol catalyst (1.0 mol%), 2.5 mmol H2O2, 5 mL model oil in H2O:
EtOH (7:3 v/v, 5 mL) at 60 °C for 2 h. The oil phase was withdrawn and analyzed by gas
chromatography (GC) for concentration of sulfur compounds. After the reaction, the reaction
mixture was cooled to 10 °C, and the catalyst was recovered by centrifugation. The products
were extracted using Et2O from the solvent phase and analyzed by GC-MS.
3. Results and discussion
The POM-IL catalysts were synthesized by two-step atom economic reaction (Scheme 2).
The zwitterionic-type precursors were prepared through a one-step direct sulfonation reaction
of pyridine derivatives and 1, 3-propanesultone. The supernatant liquid was analyzed by GC-
MS. Based on GC-MS results no by-product was produced in the zwitterions synthesis
procedure. The zwitterions acidification was accomplished by mixing of zwitterions with PW,
which led to the formation of organic-inorganic salt.
t-Bu
t-Bu
N
N
PW (aq.)
r.t. 72 h
PW (aq.)
r.t. 24 h
N
[PyPS]PW
N
[BuPyPS]PW
Py
(H2C)3
(H2C)3
BuPy
SO3
SO3
O
O
PyPS
BuPyPS
S
O
Ph
Ph
Ph
Ph
PS
PW (aq.)
r.t. 48 h
N
N
BzPy
PW (aq.)
r.t. 72 h
[PhPyPS]PW
[BzPyPS]PW
PhPy
N
N
(H2C)3
(H2C)3
SO3
SO3
PhPyPS
BzPyPS
Scheme 2. Synthesis of POM-IL catalysts.
Page 8 of 29
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The IR spectra are quite useful to find structural and bonding changes in the Keggin unit
present in the POM-ILs. The PW Keggin ion structure is well known and consists of a PO4
tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These
groups are connected to each other by corner-sharing oxygens [32]. Accordingly, PW shows
typical bands for absorptions at 1080 (P-O), 984 (W=O), 896 and 814 (W-O-W) cm-1. The IR
spectra of the POM-ILs showed that the basic structure and geometry of the Keggin anion are
preserved (Fig. 1).
Fig. 1. IR spectra of (a) PW, (b) [PyPS]PW, (c) [BuPyPS]PW, (d) [PhPyPS]PW, (e)
[BzPyPS]PW and (f) reused [PhPyPS]PW.
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9

Catalytic activities of the POM-ILs were investigated in the oxidation of MPS as a model
substrate using aqueous H2O2 at ambient temperature in water. The oxidation progress with
[PyPS]PW/H2O2 was monitored by UV-vis spectroscopy (more details were described in
experimental section). It can be seen that the absorption band at 250 nm for MPS (in n-
hexane) decreases significantly with increasing the reaction time (Fig. 2(a)). The decrease in
the absorption maximum at 250 nm (ln(A0/At)) as a function of reaction time for the organic
cations, PW and the POM-ILs are shown in Fig. 2(b). A blank experiment performed in the
absence of catalyst showed very low conversion. The counterpart cations showed remarkably
poor activities while the conversion is better for the POM-ILs. The course of the oxidation
reaction was strongly influenced by the nature of the organic cation in the POM-ILs and
decreased in the following order: [PyPS]PW > [PhPyPS]PW > PW > [BuPyPS]PW >
[BzPyPS]PW.
Solubility of the POM-ILs is the major factor that should be considered in their catalytic
applications in view of catalyst recycling. The most used term to describe solvent–solute
behavior is “polarity”. The commonly held molecular view of polarity clearly is not
appropriate to the POM-ILs. Until recently there was no systematic examination of the
complicated nature of POM-ILs/solvent interactions. It seems that the polarity and
subsequently solubility or hydrophobic/hydrophilic properties of these compounds as an
organic-inorganic POM-based salts is affected by factors including intermolecular interactions
such as van der Waals bonds, hydrogen-bonds and electrostatic interactions, the interaction of
solvent with pi- and non-bonding electrons, hydrogen bond acidity and basicity and the
dispersion force interactions. Each of above factors could be varied by changing the
temperature and features which have a significant, temperature-dependent impact on the
solubility of these POM-ILs. Solubility-temperature curves of the catalysts in water are shown
in Fig. 3. [PyPS]PW shows high solubility in water even at low temperatures, so it is not
suitable choice in the view of catalyst recycling. [BuPyPS]PW, [PhPyPS]PW and
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[BzPyPS]PW catalysts show relatively good thermoregulated solubility in water. But,
[BuPyPS]PW and [BzPyPS]PW exhibit low catalytic activities (Fig. 2 (b)). As a result,
[PhPyPS]PW was selected as the best catalyst in terms of catalytic activity and solubility. It
was insoluble below 15 °C and the solubility increases markedly when the temperature rises
over 25 °C. In addition, the reversibility of solubility is also observed. When the temperature
decreases below 15 °C, the catalyst dissolved in water precipitates again. Therefore, it may be
separated from the reaction mixture, with high recycling efficiency.
(a)
(b)
Fig. 2. (a) Monitoring of the decrease in MPS absorbance with increasing the reaction time;
(b) Comparison of the oxidation rates of the catalysts, monitored by UV–vis spectroscopy at
250 nm (MPS: 5 mmol, H2O2: 10 mmol, and catalyst: 0.5mol%, in H2O at room temperature).
Page 11 of 29
11

Fig. 3. Solubility-temperature diagrams of the POM-ILs in water.
It has been reported that various peroxo complexes are formed during the reaction, in
aqueous solutions of H2O2 with Keggin-type PW and they constitute the basis of what is
called the Venturello–Ishii catalytic system [33-36]. These peroxo species actually catalyze
the oxidation of organic compounds. The generation of active species was qualitatively
studied using UV–vis spectroscopy of the [PhPyPS]PW catalyst under different conditions
(Fig. 4). The UV–vis spectrum of the catalyst on addition of hydrogen peroxide showed
appearance of a new broad band at about 370-400 nm and the shoulder peak at 275 nm
disappeared. Further, with addition of MPS to a solution of the catalyst and H2O2, the
intensity of peak in region 370–400 nm was significantly decreased and the shoulder peak at
275 nm was observed. Based on these results, it is concluded that the reaction of
[PhPyPS]PW with hydrogen peroxide leads to the reversible formation of the active species
that catalyzes the oxidation of sulfides. However, there is still not a unanimous viewpoint for
Page 12 of 29
12

the mechanisms of oxidations by POM-IL catalysts and, hence, the nature of active species
remains unclear and it is under investigation in our laboratory.
Fig. 4.UV–vis spectra of [PhPyPS]PW catalyst in water before and during the oxidation
reaction with H2O2 (Catalyst: 10-5 M; H2O2: 5 × 10-2 M; MPS: 10-2 M).
Several studies showed that the presence of a large counter cation, structure and position of
the counter cation around the Keggin cage and interaction between the counter cation and the
POM anion influence the surface acid--basic properties of POM, kinds of active species,
reaction pathway, and subsequently mechanism of oxidation [37-42]. Cavaleiro and co-
workers [42] reported a radical mechanism for the oxidation of cyclooctane. They explained
three step mechanisms:
1. Coordination of H2O2 to the W atoms of the tetrahexylammonium (THA) salt to form
peroxo complex [W2O3(O2)4]2-.
2. Formation of one-electron reduced tungsten.
Page 13 of 29
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3. Activation of H2O2 at the tungsten atoms, by means of the generation of HO• and HOO•
radicals.
In order to test for an electron-transfer mechanism, the model reaction was carried out in
the presence of acrylonitrile (2 mmol) as a well-known radical scavenger. The product yield
was decreased from 90% to 68% under these conditions which showed that radical
mechanism is at least one of the pathways in this oxidation reaction.
The redox properties of the catalyst play an important role in the occurrence of the radical
process. The cyclic voltammograms of the POM-ILs are shown in Fig. 5. As a comparison,
the cyclic voltammogram of the pure PW is also presented. PW shows three couples of sharp
and well-defined redox waves correspond to reversible two-electron reduction/oxidation
processes of the POMs [43]. The peak potentials of [PyPS]PW and [PhPyPS]PW catalysts lie
almost at the same values as the parent PW and the redox activity of the anion is indeed
preserved in these salts. But, as displayed in Fig. 5, the cyclic voltammograms of
[BuPyPS]PW and [BzPyPS]PW catalysts show obvious differences. The initial reduction
process (I/I′) is irreversible and shifted to negative potentials. The other two reduction peaks
(II/II′ and III/III′) are reversible but the reduction peak potentials shift towards negative values
and the shift is more pronounced for [BzPyPS]PW compared with [BuPyPS]PW. Because of
the poor catalytic activities of these catalysts (Fig. 2(b)), it can be suggested that the initial
reduction process of PW plays an important role in model reaction.
The onset reduction potential for H2O2 was measured in the presence of PW, [PyPS]PW
and [PhPyPS]PW. The direct reduction of H2O2 required a large over potential at most
electrode surfaces and no obvious response was observed at the glassy carbon electrode under
our experimental conditions. On the basis of the voltammetric results, the reduction of H2O2
in the presence of the PW anion occurred at a potential about -137 mV. It is more negative
Onset
than the reduction potential in the presence of [PyPS]PW (ERed
= 197 mV) and
Onset
[PhPyPS]PW (ERed
= 185 mV). Therefore, these POM-ILs can effectively activate the
14
Page 14 of 29

H2O2 molecule and showed higher catalytic activity compared with PW (Fig. 2(b)). However,
the present results do not allow more detailed description of the reaction mechanism. More
investigations are inprogress in our laboratory, and will be reported in due course.
Fig. 5. Cyclic voltammograms for PW (5 × 10-4 M) and the POM-ILs (5 × 10-4 M) in
phosphate buffer (0.1 M), pH = 1, scan rate: 0.1 V/s, ambient temperature (20±5 °C).
Page 15 of 29
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The influence of different reaction parameters on the selectivity and efficiency of oxidation
process was evaluated in the model reaction using [PhPyPS]PW as the best catalyst (Fig. 6).
At different MPS/H2O2 ratios, we observed different relative amounts of methyl phenyl
sulfoxide (MPSO) and methyl phenyl sulfone (MPSO2) (Fig. (6a)). The optimized conditions
for selective production of each of the two compounds were determined. With equal molar
amounts of H2O2 and MPS, MPSO was produced with 94% selectivity. Using 0.25 mol% of
the catalyst to MPS in the same conditions produced MPSO as the sole reaction product (Fig.
6(b)). On the other hand, with excess amounts of H2O2 (1:4) (Fig. 6(a)) and catalyst (0.75
mol% to MPS) (Fig. 6(c)), MPSO2 appeared as the sole oxidation product at room
temperature. With these optimized results in hand, a series of sulfides was subjected to
oxidation under the specific conditions required for sulfoxide or sulfone production (Table 1).
Various types of sulfides were oxidized to the corresponding sulfoxide or sulfone with
excellent selectivity (Table 1, entries 1-7).
(a)
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16

(b)
(c)
Fig. 6. Effect of (a) MPS: H2O2 molar ratio (catalyst loading: 0.5 mol%; MPS: 5 mmol), (b)
catalyst loading (MPS: 5 mmol; H2O2: 5 mmol) and (c) catalyst loading (MPS: 5 mmol; H2O2:
20 mmol) in the selective oxidation of MPS in the presence of [PhPyPS]PW catalyst in H2O at
room temperature.
Page 17 of 29
17

Table 1. Selective oxidation of sulfides to sulfoxides and to sulfones with 30% aq. H2O2
using [PhPyPS]PW as catalyst.
Yielda (selectivity) (%)
Entry Substrate
Sulfoxideb
98 (100)
98 (100)
85 (100)
98 (100)
90 (100)
90 (100)
Sulfonec
98 (100)
98 (100)
90 (94)
S
1
2
3
4
5
6
Ph
Ph
Ph
S
S
Ph
S
98 (100)
95 (100)
95 (100)
S
S
S
7
8
75 (100)
95 (95)d
80 (94)
S
95 (100)e
9
98 (100)d
95 (95)d
98 (100)e
90 (100)e
Ph
Ph
S
S
Ph
Ph
10
a Isolated yield.
b Method (a).
c Method (b).
d Method (c).
e Method (d).
Because solid sulfur compounds (Table 1, entries 8-10) are insoluble in aqueous medium, it
was necessary to use a co-solvent that solubilizes these compounds in water. Based on
literature survey, in polar organic solvents such as acetonitrile, methanol, ethyl acetate and
acetone, the oxidation reaction proceeded rather fast, and gave mixture of sulfoxides and
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sulfones with good conversion [44-46]. Among them, methanol was consequently the solvent
of choice. In this study, EtOH was used as the co-solvent since it is derived agricultural
products and is renewable, biologically less objectionable in the environment and less toxic
compared with methanol. In order to optimize the reaction conditions, DBT, one of the major
sulfur-containing compounds present in fuels, was selected as model substrate. It showed
absorption band at λmax = 230 nm (Fig. 7(a)). We selected 7:3 v/v H2O: EtOH ratio as the
solvent composition, since higher amount of water resulted in precipitation of the DBT. The
H2O2-oxidation of DBT was not performed at room temperature. Whereas by increasing the
temperature up to 60 °C, the catalytic activity was significantly improved and DBT absorption
band decreased significantly (Fig. 7(a)). Therefore, the oxidation reaction was performed
using different concentrations of H2O2 and different amounts of the catalyst in 60 °C (Fig.
7(b)-7(d)). The corresponding sulfoxide could be obtained in excellent yield in the presence of
1 mol% of catalyst to DBT and 1:2 DBT/H2O2 ratio with trace over-oxidation product (Fig.
7c). With increasing the amounts of oxidant to 1:6 DBT/H2O2 ratio, the corresponding sulfone
was obtained with 100% selectivity (Fig. 7(d)). The obtained results showed that the solid
substrates were completely oxidized to the corresponding sulfoxide and sulfone under these
optimized conditions (Table 1, entries 8-10).
Based on the above results, the catalytic system was used to ODS of model oil containing
DBT (500 ppm), MPS (250 ppm) and thiophene (250 ppm) in 5 mL of n-decane with the total
sulfur concentration 1000 ppm. At the end of the reaction, the upper phase was withdrawn and
analyzed by GC. The sulfur in diesel fuel was reduced from 1000 ppm to 64 ppm (93.6%). In
order to investigate the deep desulfurization efficiency of the catalyst, the catalytic system was
used to ODS of model oil with low concentration of DBT (100 ppm level) in n-decane. The
sulfur level was lowered from 100 to 0.8 ppm after ODS process. The remaining hydrogen
peroxide was determined by CeVI(SO4)2 (0.1 M) titration [47] and the efficiency of H2O2
utilization was calculated according to the following formula: [(2 × mol (DBT))/(mol (H2O2)
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19

consumed)]×100. The direct decomposition of hydrogen peroxide was observed during the
oxidation process, but the efficiency of H2O2 was higher than 90% for desulfurization of this
model oil (DBT = 100 ppm).
(a)
(b)
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20

(c)
(d)
Fig. 7. (a) Monitoring of the decrease in DBT absorbance with increasing the reaction time
(DBT: 5 mmol, H2O2: 20 mmol, and catalyst: 0.5 mol%, in H2O: EtOH (7:3 v/v, 10 mL) at 60
°C); Effect of (b) DBT: H2O2 molar ratio (catalyst loading: 0.5 mol%, DBT: 5 mmol), (c)
catalyst loading (DBT: 5 mmol, H2O2: 10 mmol), and (d) catalyst loading (DBT: 5 mmol,
H2O2: 30 mmol) in the selective oxidation of DBT in the presence of [PhPyPS]PW catalyst in
H2O: EtOH (7:3 v/v, 10 mL) at 60 °C.
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Easy recyclability and the prolonged retention of catalytic activity are very important
features for practical applications of catalytic processes. [PhPyPS]PW catalyst has combined
the advantages of a one-phase homogeneous catalysis with an easy way of catalyst separation.
It was used in four 20 min catalytic cycles of MPS oxidation with H2O2 (Fig 8 (a)). After the
reaction, the mixture was cooled to 15 °C and the catalyst was separated by filtration after
each cycle, washed with ether and dried at 50 °C. Five consecutive preparation of MPSO
showed no significant loss in yield. After five cycles, the catalyst was recovered and weighed.
The loss of 12.5% was found, which is agree with the calculated amount according to Fig. 3
(the solubility of ionic liquids, 0.113 mg/mL at 15 °C) and the used solvents amount (2 mL) in
each cycle. If the temperature is reduced to 10 °C after the each reaction cycle, the loss of the
catalyst can reduced to 9.3% accordingly. When the equivalent amount of lost catalyst is
added, the yield can be restored to 98%.
Reusability of [PhPyPS]PW catalyst was also investigated in model oil. After the reaction,
model oil was transferred from the reaction vessel by separator funnel and the aqueous phase
was cooled to 10 °C. The catalyst was separated by filtration, washed, dried and used in the
next ODS reaction. The oil phase was analyzed by GC for concentration of sulfur compounds.
The products were extracted by Et2O from the solvent phase and analyzed by GC-MS.
Corresponding sulfones have been obtained with excellent selectivities in all runs. The results
indicate that the catalyst can be recycled five times with a slight decrease in desulfurization
(Fig. 8 (b)). The IR spectrum of reused catalyst confirmed the existence of Keggin structure
after five cycles (Fig. 1 (f)).
Table 2 shows the comparison of catalytic activity of [PhPyPS]PW catalyst with some
reported POMs catalyzed H2O2-based sulfoxidation reactions. As can be seen, [PhPyPS]PW
catalyst shows considerable activity with unique thermoregulated phase-separable behavior in
comparison with the others.
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22

(a)
(b)
Fig. 8. Reusability of [PhPyPS]PW catalyst in (a) oxidation of MPS (MPS: 5 mmol, H2O2: 5
mmol, and catalyst: 0.25 mol% in H2O at room temperature) and (b) oxidative desulfurization
of model oil (DBT: 100 ppm in n-decane, H2O2: 2.5 mmol, and catalyst: 1.0 mol% in H2O:
EtOH (7:3 v/v, 5 mL) at 60 °C for 2 h).
Table 2. Comparison of the reaction data with other reported methods.
Substrate
MPS
Catalyst
Reaction conditions
Yield (%)
98
[PhPyPS]PW
H2O2, H2O, r.t., 20 min
CinH3PMo12O40, (Cin =
cinchonine)
MPS [48]
MPS [7]
MPS [49]
H2O2/urea, EtOH, r.t., 2 h
95
PyH3PMo11VO40
H2O2, CH3CN, r.t., 3 h
H2O2/HClO4, CH3CN/tert-
BuOH, r.t., 2h
100
99
[γ-PW10O38V2(μ-OH)2]3−
DBT(1000 ppm)
DBT(1000 ppm)
[PhPyPS]PW
[PhPyPS]PW
H2O2, H2O/EtOH, 60 °C, 2h
H2O2, H2O/EtOH, 60 °C, 2h
93.6
99.2
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DBT (500 ppm) [50] H3PW12O40/TiO2
H2O2, CH3CN, 60 °C, 2h
H2O2, 60 °C, 2h
95.2
100
DBT (500 ppm) [26] [Bmim]3PMo12O40/SiO2
DBT (500 ppm) [25] [MIMPS]3PW12O40 2H2O
H2O2, [Omim]PF6, r.t., 1 h
99.5
4. Conclusions
In agreement with the ever increasing environmental awareness, we developed a new
reusable catalytic system for sulfoxidation reaction based on temperature-dependent solubility
of the POM-ILs. This catalyst has been successfully applied for selective oxidation of
thioethers and thiophenes to the corresponding sulfoxides and sulfones with dilute H2O2, in
water or EtOH: water biphasic system. The results showed that the catalyst has exhibited not
only excellent redox property of POMs but also good recycling efficiency. Catalyst/product
separation was very simple and the catalyst was recovered upon cooling of the reaction
mixture and reused several times without obvious loss of catalytic activity. This catalyst also
showed excellent activity for the removal of sulfur contents in model oil. So, it has a high
potential as an effective catalyst for deep desulfurization of real diesel. We hope that the
knowledge gained in this work will lead to the use of thermoregulated phase-separable POM-
IL catalysts in a wide variety of organic reactions in aqueous media.
Acknowledgements
The authors thank the Razi University Research Council for support of this work.
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Highlights
 Novel polyoxometalate salts were synthesized and characterized.
 They were used as thermoregulated phase-separable catalyst for ODS process.
 They showed characteristic of homogeneous catalysis and heterogeneous separation.
 The process was successfully used for deep desulfurization of model oil.
 There was no considerable loss in catalytic activity after several reaction cycles.
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*Graphical Abstract (for review)
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